Ultra-wide band (UWB) technology also referred to as impulse, baseband, and zero-carrier technology, uses ultra short pulses, typically less than a nanosecond in duration, to convey information. The ultra short pulse duration means that the signal is spread across a wide bandwidth, which typically exceeds one-quarter of the nominal center frequency. Since the distribution of energy is spread across a wide spectral range, the spectral density is very low.
UWB communications systems have been described as the most promising technology to emerge from the telecommunications industry in recent years. The reason is that UWB technology has several unique features, such as immunity to multi-path interference, immunity to jamming and interference, low probability of detection, low power consumption, and enhanced penetration capability, that make it attractive for use in communications systems. In addition, UWB technology is attractive for communications because the elimination of radio frequency (RF) components in UWB systems allows for the use of transmitters and receivers with relatively low hardware complexity.
Since UWB transmitters emit signals at levels below the noise floor, UWB signals have a low probability of detection and a low probability of interception. While these properties are desirable for covert communications and may cause minimal interference on licensed/unlicensed bands, they make it difficult to demodulate and decode the signal. Fortunately, spread spectrum techniques are well suited to extract UWB signals under these circumstances. In spread spectrum techniques, the frequency components of the signal are “spread” across the frequency spectrum by encoding each bit of information in a symbol consisting of a series of “chips” that are transmitted during a symbol period that is allotted for each bit of information.
Multiple channels may be enabled to operate simultaneously through the use of either Direct Sequence Code Division Multiple Access (DS-CDMA) or Time Hopped Code Division Multiple Access (TH-CDMA) where each channel is assigned a code sequence c={c1, c2, . . . , cNc}. During each symbol period, a sequence of Nc chips is transmitted. Let d denote the duty cycle (i.e., fraction of pulse duration over a chip period). Given a bandwidth W, a DS-CDMA or TH-CDMA system with spreading factor Nc allows symbol rates up to d*W/Nc.
One particularly challenging issue for a UWB communications system employing DS-CDMA with antipodal signaling is adequately resolving the polarization of the received signals. There are some applications for which UWB is targeted to operate in harsh environments (e.g. military or emergency rescue). In such environments, the transmitted pulse shape can be severely distorted to the point where polarization resolution becomes particularly challenging.
TH-CDMA signals typically are modulated using pulse position modulation (PPM). TH-CDMA allows for channelization via time-hop sequences and PPM eliminates the need for resolving the polarization of the received signal. However, the number of possible orthogonal time-hop sequences is severely limited. If the number of orthogonal time-hop sequences is not sufficient, a random (or pseudorandom) hopping sequence may be used. However, such sequences do not guarantee orthogonality between channels and the receiver encounters higher multiple access interference. As a result, neither DS-CDMA with antipodal signaling nor TH-CDMA is particularly well suited for UWB operation due to harsh environments and the limited number of orthogonal hopping sequences, respectively.
Thus, there is a need in the art for a modulation scheme that allows for the use of a large number of orthogonal codes, yet operates in harsh environments without the need to resolve polarization.